It seems like Bavix\Helpers\Arr::get($...>matches, 'ext', 'php') can also be of type array; however, parameter $key of Bavix\Helpers\Arr::get() does only seem to accept string, maybe add an additional type check?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation